There have been a few attempts at Route Optimization (RO) solutions developed in the wireless industry. Recently, the notion of route optimization has become synonymous with Mobile IPv6. Mobile IPv6 as defined in RFC 3775, which is hereby incorporated by reference, includes details about using Return Routability (RR) for route optimization. However, there are some significant drawbacks in doing route optimization only as defined in Mobile IPv6 RFC 3775.
First, it is a mobile node centric solution. A mobile node can be a cell phone, a personal digital assistant (PDA), a wirelessly enabled laptop or computer device, or any other applicable device. The initiation of route optimization and the need for optimizing the route with a correspondent node is decided by the mobile node. However, the mobile node has no knowledge of either the routing policies of the network or the network layout. So, the mobile nodes are likely to initiate return routability procedures without knowing whether there will be any difference in the way the IP packets are actually going to be routed.
Second, the network has little or no say on whether route optimization is necessary or whether it is even allowed for certain applications. For example, if the operator has a policy that requires the route to be via a certain network segment or certain nodes (for packet inspection or security and charging policy enforcement), there is no standardized way to allow or disallow the mobile node to initiate route optimization on a per application basis.
Third, the route optimization solution with Mobile IPv6 only works for nodes with either dual stack or IPv6 routing infrastructure in the network. It is costly for operators to completely switchover to an IPv6 network at one time. IPv6 deployment will take time to implement and route optimization should not be tied to only IPv6 capability in the operators' networks.
Fourth, inter-access gateway (AGW) handoff management when mobile initiated route optimization is in use is very time consuming. The mobiles have to perform return routablity (6 messages) every time there is an inter-AGW handoff to inform the other end that a change of care-of address (CoA) has occurred.
Fifth, mobile node based route optimization technique requires over-the-air signaling. For mobile-to-mobile calls, this is about 12 messages. per session. For AGW handoffs these have to be repeated. This not only adds over-the-air overhead, but also increases signal processing burden for the mobile and the network nodes.
Finally, even if mobile based route optimization is in use, there is a moment when VoIP packets traverse the reverse tunnel. This occurs because the mobile node reverse tunnels the packets to a home agent before return routablity is complete.
The statistics gathered from circuit voice networks show that the majority of the voice calls made in a region remain within the region (i.e., the calls are highly localized). In some of today's wireless networks, call sessions are routed to a home agent device that tracks where a mobile node is in the network. The calls in Mobile IP based networks often send IP packets to a home agent from the local radio access network in a reverse tunnel. This home agent may not be located geographically close to where the call session originates at the mobile node. If reverse tunneling is not used, it results in triangular routing among the mobile node, a correspondent node (i.e., the second device involved in the call session), and the home agent.
The use of reverse tunneling simplifies the network resource management, policy, and charging enforcement at a centralized location such as the home agent. However, for conversational real time applications, the reverse tunneling causes additional data transport latency which may vary depending on the distance traveled and the number of devices involved (i.e., backhaul link type). The sending of packet data to a home agent also requires the operators to increase backhaul link bandwidth to carry the data traffic for these applications whether or not reverse tunneling is used.
A tunneling protocol, which is used for reverse tunneling and sometimes for sending packets, encapsulates one protocol inside another protocol. Tunneling protocols also provide a mechanism to transport packets between two endpoints over a foreign network.